For producing hollow plastic articles with rotationally symmetric cross section along their entire length and their entire height, respectively, e.g. round bottles, an extruded tube is generally used having a diameter which corresponds for example to the diameter of the neck of the hollow article to be produced and a wall thickness which exceeds the wall thickness of the finished hollow article. The tube is blown up in a blowing mold to the finished hollow article without experiencing an irregular wall thickness distribution.
For hollow articles with a cross section deviating from the circular cross section like e.g. flat bottles, a production in this manner is not applicable as correctly reasoned in detail in the US-PS No. 3,892,829. Therefore, the mentioned US-PS provides a separate intermediate mold station in which an intermediate form is produced the length of which being considerably smaller than the height of the flat bottle and the surface of which being 25% smaller than the surface of the flat bottle to be produced. Only then can it be prevented during shaping of the intermediate form to the finished flat bottle in a final blowing mold that parts of the intermediate form are squeezed between both mold halves of the final blowing mold during their closing and that so-called dog-ears are obtained. The thus necessary biaxial stretching of the plastic material leads, however, to an irregular wall thickness distribution over the circumference of the flat bottle because the stretching occurs exclusively in the surface areas which do not yet bear against the cooled inner wall surface of the final blowing mold, thus preferably in the area of the narrow sides and the bottom of the flat bottle. In order to maintain a sufficient wall thickness, it is necessarily required to work with a surplus of material. The finished flat bottle has therefore an unnecessarily great wall thickness in the area of both its broad sides. Apart from the thus redundantly great material consumption such an excessive wall thickness results in an extended cycle period since evidently the cooling period is dependent on the area with the greatest material strength. This fact is in particular relevant when the intermediate form is not produced from a preform which is injected-molded around a hollow mandrel but is made from a freely extruded tube since the tube end sealed through pinching is arranged in the bottom of the intermediate form and the flat bottle, respectively, and causes there an especially high material accumulation. The method could not succeed in practice. Rather, the production of flat bottles with or without grip is usually accomplished through a tube blowing method in which an extruded tube is used with a circumference nearly equal to the greatest circumference of the flat bottle to be produced. The tube is continuously extruded in suspended position with a wall thickness desired for hollow articles and, after attaining a length corresponding to the height of the flat bottle, is transferred to a blowing mold in which the tube-like preform is blown up to the final flat bottle by means of a blow pin. The blow pin simultaneously calibrates neck and mouth of the bottle while the blowing mold during closing has already pinched off the considerable extra material portion especially in the neck area and shoulder area.
Since depending on the shape of the flat bottle the pinched off material portion may amount from 10% up to 25% and more of the weight of the finished bottle, this type of production is relatively uneconomic since the need of machinery, especially, however, the power demand for drive, heating and cooling of the extrusion blow-molding apparatus is considerably higher than is necessary for the production of the actual flat bottle because the pinched-off material portion, too, must certainly be liquefied at first and then also cooled and a separate punching apparatus for its removal from the flat bottle as well as transport apparatus and regenerating apparatus for reprocessing the waste material are required.
The DE-PS 27 20 448 discloses an apparatus for production of flat bottles which includes a blowing mold essentially provided with two mold halves wherein each mold half includes two mold parts movable perpendicular to the parting plane of the mold with one mold part forming the neck, the narrow sides and the bottom of the flat bottle, and the other mold part forming the broad sides of the flat bottle. A tube-extruded or injection-molded preform is used with a diameter approximately equal to the neck diameter of the flat bottle. The preform is blown up to the flat bottle during closing of the mold wherein the closing step proceeds in such a manner that initially the respective first or outer mold parts of both mold halves are closed. This material-saving process requires, however, a very precisely controlled blowing action. Further, the problem of undesired material accumulation in the bottom of the flat bottle still cannot be overcome when using an extruded preform. The method results in an essentially uniform wall thickness only in flat bottles whose radii of curvature is not too small because in these cases the material will still be stretched in the respective edge areas and corner areas upon already completely closed mold. It should be considered, however, that a relevant part of flat bottles used in the market and demanded from fillers have relatively sharp edges and thus small radii of curvature. In view of its costs, an blow-molding apparatus must basically, i.e. after installation of the respective blowing molds, be capable to produce all conventional flat bottles.